The main objective of this project is to determine the conditions necessary for immunologic activation of leukemia viruses and study the mechanism of C-type virus activation by the immune system. In the first 8 months, assays to detect bovine leukemia virus were developed and standardized. Isopycnic centrifugation analysis of H3-uridine labeled bovine leukemia virus revealed virus peaks at a density of 1.16 g/cm3. This assay was found quite sensitive in detecting bovine leukemia virus activated by phytomitogen treatment of lymphocytes from lymphocytotic cows or propagated in fetal lamb spleen cell cultures. RNA-dependent DNA polymerase (RDDP) assay was also standardized using Rauscher strain of murine leukemia virus. RDDP activity was detected in purified bovine leukemia virus in the presence of Mg ions, and poly rA. dT12-18. It was found that 10 mM of Mg ions was an optimum concentration for maximum RDDP activity of bovine leukemia virus. Lipopolysaccharide, a B cell mitogen in the mouse was mitogenic for lymphocytes of normal cows and activated bovine leukemia virus from the lymphocytes of a lymphocytotic cow.